Recent receptive field mapping studies of primary somatosensory cortex (SI) in cat and monkey have demonstrated that the body surface is represented as a mosaic of discrete placed-defined cortical columns, "segregates", approximately 0.3-0.6 mm in diameter. The neuroanatomy of these cortical modules will be studied in cats and rats: Small injections of retrograde tracers, including gold-labeled wheat germ agglutinin conjugated to apo- horseradish peroxidase (WGA-apoHRP-Au) and various fluorescent tracers, will be made in individual physiologically-identified SI segregates to establish their thalamic input. Similar experiments will be performed using neurotropic viral tracers to permit transsynaptic retrograde labeling. In this way, the disynaptic pathways to the cortex from the dorsal column nuclei and the spinal dorsal horn will be studied. The organization of inhibitory intracortical connections of electrophysiologically-defined segregates will be studied using the transmitter-selective retrograde tracer [3H]GABA, and by combining GABA immunocytochemistry with WGA-apoHRP-Au pathway tracing. Retrograde transport of the transmitter-selective tracer [3H]D-aspartate will be used to study the organization of excitatory intracortical connections; immunocytochemistry with antibodies for glutamate and aspartate will be used to further define the transmitter used by these neurons. This analysis of the modular connectivity of SI directly addresses fundamental principles of cortical organization.